1. Field of the Invention
This invention relates to improvements in dense crown optical glass.
2. Description of the Prior Art
The conventional dense crown optical glass basically comprises a SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --BaO system. Of the dense crown optical glasses, the glass in the range of high refractive index and low dispersion contains a great deal of BaO, and, therefore, has a very poor chemical durability. Along with the advance of optical glass polishing and machining techniques, optical glasses have become often exposed to severe machining conditions, and along with the spread of multi-layer coatings, the presence of a surface corrosion-degeneration layer has become impermissible. Thus, more importance has become attached to the chemical durability of optical glasses than before. To improve this, there is an optical glass having La.sub.2 O.sub.3 introduced in said system. Such optical glass is disclosed, for example, in U.S. Pat. No. 3,248,238. This glass comprises the following composition in percent by weight:
______________________________________ SiO.sub.2 24-32 B.sub.2 O.sub.3 16-27 BaO 15-40 BaO + ZnO + MgO + CaO 38-44 La.sub.2 O.sub.3 3-7 ZnO + CaO + MgO 0-26 ______________________________________
However, it cannot be said that this optical glass has a fully improved chemical durability.